Make stdout debug level configurable

[elgalu]

Hi, I'm currently getting a lot of debug info in x11vnc logs in my project https://github.com/elgalu/docker-selenium.

It would be nice to be able to configure the log output level, for example to only show a minimal output like when the VNC server is ready & listening for connections.

[bk138]

Feel free to provide a pull request!

[elgalu]

Hehe fair enough, I'm too rusty on C or Tcl to do that but who knows maybe some hacker jumps in with a PR :P